![]() ![]() It may not be easy to train a patient or technologist that the breath-hold CT to best match with PET should be taken at the mid-expiration in light breathing not in deep breath in and deep breath out as shown in Fig. It was because interpretation of a mid-expiration position can vary from patient to patient. In our study of 100 patients coached to hold breath at mid-expiration, 50 patient data sets exhibited a misregistration of more than 1 cm between the CT and PET data. However, the outcomes were mixed because coaching the patient to hold breath at a certain state during CT data acquisition is not very reliable from both the perspectives of the patient and the technologist operating the PET/CT scanner. It is this mismatch in temporal resolution between the CT and PET data acquisitions that causes misregistration between the CT and PET images in PET/CT imaging of a patient at free breathing.Ĭoaching the patient to hold breath at mid-expiration during the CT acquisition was suggested as an alternative to improve the registration between the CT and PET data. There is no guarantee that the CT images from a fast scan CT at the temporal resolution of sub-second would match temporally with the PET images of a slow PET scan. The duration of a PET scan at each bed position is typically 2–3 min, and it can be shortened by utilizing a larger axial field of view such as the 21.6 cm detector on the Siemens mCT scanner to further improve the sensitivity of PET imaging. The PET detectors are arranged in a ring geometry without space or gap to improve also the sensitivity. The scan coverage of a GE PET detector module in the craniocaudal direction is 15.4 cm, much larger than the CT detector module of 2 cm for the 16 slice or 4 cm for the 64 slice. Most of the PET acquisitions today are in the 3D acquisition mode without the septa to improve the sensitivity of detecting annihilation photons of 511 keV. 16.3, where the respiratory and cardiac motion artifacts are highlighted. An example of the inconsistence is illustrated in the sagittal view CT image of a patient in Fig. The maximum and minimum amplitudes are by convention assigned to end-inspiration and end-expiration phases, respectively. End-inspiration phase is 0 % and end-expiration phase is around 40–60 %. On the other hand, end-expiration phases correspond to a local minimum in amplitude. A phase is determined as a percentage in a breath cycle between two consecutive end-inspiration phases, which typically correspond to a local maximum in amplitude. However, these 2D CT images, almost free of respiratory motion artifacts, cannot guarantee consistency of the 3D CT anatomy in the craniocaudal direction because each CT image is a snapshot of the anatomy at a particular phase of a respiratory cycle when the patient free-breathes during the CT data acquisition of a PET/CT scan. Fast CT gantry rotation time of sub-second improves the temporal resolution of the CT image to sub-second, making the CT image almost free of respiratory motion artifacts. The 3D volume of a CT scan is composed of a stack of 2D CT images acquired in the craniocaudal direction. On the other hand, combination of a slow gantry rotation cycle time of 1 s, a smaller X-ray collimation of 1 cm for a 16-slice or 2 cm for a 64-slice CT, and a pitch factor of 1 would make the scan speeds of 1 and 2 cm/s for a 16-slice and 64-slice CT, respectively. A faster CT scan can be accomplished with a fast gantry rotation cycle time of 0.5 s, the X-ray collimation of 2 cm for a 16-slice or 4 cm for a 64-slice CT, and a pitch factor of 1.375 for the scan speed of 5.5 and 11 cm/s for a 16-slice and 64-slice CT, respectively. The speed of helical CT is determined by the following three components: gantry rotation cycle time, width of X-ray collimation, and pitch factor defined as the ratio of table translation distance per rotation to the width of X-ray collimation. 1 A helical CT scan of 100 cm from the orbit to the mid-thigh in a typical PET/CT scan can be performed in less than 20 s on a 16-slice or less than 10 s on a 64-slice PET/CT. The detector coverage along the craniocaudal or table direction at the imaging center or isocenter is 2 cm for a 16-slice or 4 cm for a 64-slice GE CT scanner. ![]() A CT scanner is composed of an X-ray source and a bank of detector modules arranged in fan beam, which needs to rotate at least 180° plus 60° fan angle to produce a tomographic image. CT is mostly a 2D whereas PET is a 3D imaging device. ![]()
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